Determining a rock’s hardness is a fundamental step utilized by geologists and hobbyists in both the field and the lab. This physical property refers to a material’s resistance to abrasion or scratching, providing insight into its internal atomic structure and composition. The primary technique relies on the comparative principle that a harder substance will always leave a mark on a softer one. Measuring this characteristic allows for the reliable classification of unknown rock and mineral samples against known standards.
Understanding Mineral Hardness and the Scale
Mineral hardness measures the strength of the chemical bonds within a material’s crystal structure as they resist surface disruption. This property is solely about resistance to scratching, not the material’s ability to withstand shattering or crushing. A separate property, known as tenacity, describes a mineral’s resistance to breaking or deformation; thus, a very hard substance can still be quite brittle.
The relative resistance to scratching is quantified using the qualitative Mohs Scale of Hardness. This scale ranks minerals from 1, representing the softest material (Talc), to 10, representing the hardest natural substance (Diamond). The scale is built upon ten common minerals used as standards.
A mineral with a higher Mohs number will scratch any mineral with a lower number. For instance, Quartz (rated 7) will scratch Calcite (rated 3), but cannot be scratched by it. The scale is not linear; the difference in absolute hardness between consecutive numbers varies across the scale. The jump in hardness from Corundum (9) to Diamond (10) is far greater than the difference between Talc (1) and Gypsum (2).
Reference Materials Used for Testing
Since carrying the ten reference minerals is impractical, the scratch test relies on common, readily available materials with established hardness values. These everyday items serve as known standards to bracket the hardness of the unknown rock sample. The softest standard used is a human fingernail, which registers a hardness of 2.5. Any mineral scratched by a fingernail, such as Talc or Gypsum, has a value less than 2.5.
A common copper penny provides the next reference point, possessing a hardness near 3.5. If a specimen is not scratched by a fingernail but is marked by a penny, its hardness is between 2.5 and 3.5. For intermediate values, a steel pocket knife blade or a piece of window glass are standards, both typically sharing a hardness of about 5.5. This single reference point is useful because many common minerals, like Feldspar, fall near this range.
Hardened steel, such as a common file or a masonry drill bit, registers a hardness of 6.5. This tool helps distinguish between minerals that can scratch glass and those that cannot, such as Quartz (rated 7). By testing a rock against this series of known objects, it is possible to narrow the specimen’s Mohs hardness down to a range of 0.5 points.
Executing the Scratch Test Procedure
The scratch test begins with careful preparation of the rock specimen to ensure an accurate result. Locate a clean, fresh surface that is not weathered or covered in residue. Using a magnifying glass helps confirm the surface is smooth and free from pre-existing scratches.
Hold the reference material firmly. The reference point is pressed firmly against the rock surface, and a deliberate, short line is drawn across the sample. Apply significant pressure, especially when testing harder materials, to ensure a true attempt at abrasion is made.
The next step involves careful interpretation of the mark left on the specimen’s surface. Wipe away any powder or residue with a finger or cloth. This confirms if a true scratch—a permanent groove etched into the material—was made, or if the mark was merely residue left by the softer testing tool.
If the reference material created a groove, the specimen is softer than the tool, and a harder reference object must be used next. Conversely, if the specimen remains unmarked, it is harder than the testing tool, requiring a softer tool for the next attempt. For example, if a rock is scratched by a knife (5.5) but not by a penny (3.5), its hardness is bracketed between 3.5 and 5.5.